In the 1980s, a fixed filter was commonly used to filter and combine different aerial signals onto 1 cable. Fixed, meaning the installer needed to replace the filter by a totally new filter if frequencies of the channels (or simply the whole application) changed.
In the 1990s, programmable filters came on the market, which could be reprogrammed by the installer on site to serve different frequencies or new applications, so obviating the need for replacement by a new filter. The filter was suitable for all possible applications at that time, so there was no need to develop, produce and store different versions.
One example of such a filter is described in GB-A-2272341. In order to filter the desired TV channels from the incoming signal, the device comprises a plurality of frequency determining elements (mostly varicaps) which are settable by means of analog setting voltages. In order to generate these voltages, digitally stored voltage values are converted to analog voltages by means of Digital-to-Analog Convertors (DAC), which are expensive components. To limit the number of DACs, a special algorithm is used in the device of GB-A-2272341: a smaller amount of DACs generates more analog voltages by multiplexing each DAC-output to several “analog memory locations” (which is for instance a sample-and-hold circuit) in a “dynamic memory”, where the analog voltages are stored and passed on to the frequency determining elements.
The device known from GB-A-2272341 however has the disadvantage that, in order to keep the voltages in the dynamic memory at the desired level, a continuous refresh-algorithm is needed to connect the DAC-output at regular times to the analog memory locations. This continuous refreshing of the analog voltages demands a huge portion of the resources of the microcontroller. This results in the need of an oversized microcontroller, which again increases the cost of the device.